Small internal combustion engines, such as four stroke engines, are frequently found in consumer and light industrial products such as lawnmowers, electrical generators, power-washers, snow throwers and lawn tractors. A common problem associated with consumer or jobsite use of these types of products results from the fact that many operators do not perform the easy, but necessary, periodic inspections of the engine to ensure it contains a sufficient level of oil for operation. In many cases, checking the oil level in an internal combustion engine such as is found on a consumer or jobsite product is as simple as extracting a dipstick from the engine and visually observing how far up the dipstick the oil wet line appears. A typical dipstick is engraved with a mark or marks that indicate the appropriate level of oil. In some cases, the dipstick is marked with the words “ADD” as an instruction to operators to add oil should the observed level of oil be found to be below a certain level mark on the dipstick.
Despite the fact that checking and adding to the oil level in an internal combustion engine is a relatively easy process, operators often fail to adequately maintain the necessary level of oil required by these engines. When an engine operator repeatedly fails to protect against or recognize a low level of oil in the engine, the engine will suffer premature wear. In extreme cases of neglect, catastrophic engine failure can result. It is therefore desirable to have a low-level fluid sensor built into the engine which will alert the operator via a visible or audible signal, or more preferably prevent or shut down operation of the engine, should the lubricant level fall to a dangerously low level.
The prior art discloses a variety of methods for indicating or responding to a low fluid level or condition. Some systems employ an indicator light that turns on when an engine fluid level or pressure drops below a predetermined value. In other systems, an audible alarm may be implemented such that the operator will hear the alarm upon attempting to start the engine. In other cases, the sensing system disrupts the engine ignition circuit and protectively shuts down the engine.
The typical four-cycle internal combustion engine found on a consumer or light industrial product utilizes a splash lubrication system to spread oil throughout the engine. Accordingly, in the past various types of mechanisms have been considered for use with splash lubricated engines to detect a dangerously low lubricating fluid level. Such prior art detecting mechanisms include pressure switches, capacitor switches, thermal switches and float-type sensors. Though pressure sensors have found effective application in industrial and automotive engines, and have even been proposed for use in splash lubricated engines, such sensors have been found not practical for use in consumer product engines. Capacitance, pressure and thermal switches generally are relatively costly and require an external power supply for operation. Additionally, oil capacitance may vary by several orders of magnitude based upon oil temperature and condition and the presence of oil additives and impurities.
A variety of float-type sensors have been proposed in the prior art. The simplest version involves slidably mounting a float on fixed linear guides and having an elongate flexible member pass through the center of the float. When the float moves along the length of the fixed linear guides due to its buoyancy, the flexible member is caused to twist. The top of the central flexible member is connected to a needle on a dial. The twisting action of the central flexible member is translated into a dial readout. In order for this prior art device to function, the float must be restricted from rotating about the central flexible member so that its only motion is vertical. This is accomplished through use of the fixed linear guides. Accordingly, these float-type sensors comprise additional structures, usually fixed linear guides, located at the perimeter of the float, that prevent the float from rotating.
In more recently proposed float-type sensors, the float is provided with a magnet or electrical contact. In one variant of this type of sensor, an electrical contact is attached to a float, which rides on the surface of the lubricant in the engine lubricant reservoir or crankcase. Due to its buoyancy, the float moves up and down directly with the level of the lubricant. As the lubricant volume drops so does the float. At a point in its vertical travel designated to represent a low lubricant level, the float's vertical travel will cause the electrical contact on the float to make contact with a circuit terminal. This terminal contacting action will sound an alarm or ground the engine ignition circuit.
In another variant of a prior art float-type sensor, a magnet is housed in the float. The float rides on the surface of the lubricant in the engine lubricant reservoir or crankcase. Due to its buoyancy, the float moves up and down directly with the level of the lubricant. As the lubricant volume drops so does the float. At a point in its vertical travel designated to represent a low lubricant level, the float's vertical travel will cause the float to pass a switch connected to a circuit. The switch is activated by proximity of the magnet. Activation of the switch will sound an alarm or ground the engine ignition circuit.
Though desirable, prior art float-type sensors systems exhibit several problems. First, they suffer from the fact that they are rather large and difficult to adapt to existing equipment and due to the substantial number of moving parts are not completely failsafe. Additionally, these sensors are susceptible to giving false readings particularly when used in conjunction with a splash lubrication system such as is found in a consumer product internal combustion engine. In a splash lubrication system, a slinger gear or paddle splashes lubricant throughout the engine housing while the engine is running. This can create lubricant level sensing difficulties when starting the engine. Upon engine start-up, much of the lubricating fluid is splashed throughout the engine housing. This may cause a temporary condition wherein the level of pooled lubricant in the reservoir is very low, yet the overall amount of engine lubricant is satisfactory.
The susceptibility of the prior art float-type sensor to register an erroneously low level of lubricant is not limited to situations of engine start up. Due to its buoyancy, the sensor float height varies as the lubricant level in the crankcase fluctuates. Such fluctuations may occur during normal engine operation, for instance in a lawnmower when the lawnmower is used on steep or undulating grade. Operating the lawnmower on such terrain causes the level of pooled lubricant in the reservoir or crankcase to vary substantially within the crankcase. This variance, in turn, causes the lubricant slinger or paddle to move varying and at times excessive amounts of lubricant from the reservoir to the engine housing, which may further exacerbate lubricant turbulence.
An external indicator or automatic shutdown feature on an engine or other mechanical device is desirable in order to avoid damage caused by an insufficiency of a vital fluid. In the case of a consumer product engine, the simple closure of a circuit such as would ground the ignition circuit is a cost effective and reliable way to avoid damage due to fluid insufficiency. However, if a prior art float-type sensor is utilized, fluid level variance may cause the circuit to be alternately closed and opened. This alternate interruption and restoration of ignition will cause the engine to misfire, which is objectionable from an emissions aspect and can also result in damage to the engine. Consequently, one problem with the float-type sensing system of the known prior art is that it may register a dangerously low level of lubricant (and erroneously ground the ignition circuit) even though the amount of lubricant in the engine is sufficient.
In an effort to avoid such errant low-level readings, engine manufacturers have devised various devices such as timer circuits to delay the indication of a low oil condition for a preset period of time due to engine starting or uneven terrain. A significant disadvantage of these time delay devices is that they typically require relatively complicated and expensive circuitry, which may not be feasible for internal combustion engines or other devices intended for consumer or light industrial use. Accordingly, there is the need in the art for a float-type low-level fluid sensor that accurately gauges engine lubricant level under start-up, normal operating conditions, and also during conditions of lubricant turbulence without the use of external electrical latching circuits.